1. Field of the Invention
The present invention relates to semiconductor devices and methods of forming the semiconductor devices. More particularly, the present invention relates to non-volatile memory devices and methods of forming the non-volatile memory devices.
2. Description of the Related Art
Non-volatile memory devices, such as flash memory devices, generally include a cell array region and a peripheral circuit region for driving the cell array region. In the cell array region, programming and erasing of the cells may be performed using Fowler-Nordheim (FN) tunneling. More particularly, programming is performed by injecting electrons into a floating gate and erasing is performed by ejecting electrons from the floating gate using FN tunneling. A cell transistor in the cell array region includes a gate insulator, a floating gate, an intergate dielectric and a control gate sequentially stacked in the order named. To enhance an operating speed of a semiconductor device, control gates are made of metallic substance, e.g., tungsten silicide. However, with the continuous trend toward higher integration and higher densities of semiconductor devices, line-widths are trending below a nanometer in width. Thus, electrical sheet resistances of the conventional silicon silicide are increasing, causing increases in RC signal delays relative to distance and deterioration of cell threshold voltage distribution.
Different approaches for overcoming the foregoing problems have been proposed. One of the approaches proposed forming a control gate from a metal having a low sheet resistance, for example, tungsten. Tungsten is, however, easily corroded by a mixture of ammonium hydroxide, hydrogen peroxide and de-ionized water (DI water), which is used to remove etch byproducts. In the following description, such mixtures of ammonium hydroxide, hydrogen peroxide and DI water will be referred to as “SC1”. Accordingly, there is a need for solutions for removing etch byproducts that do not corrode such metals, e.g., tungsten. Known solutions with a lower detergency than SC1 are incapable of fully removing etch byproducts. Moreover, during an oxidation process for curing etch damage of a gate, the tungsten of a control gate is oxidized, thereby forming an unwanted body. Due to the unwanted body, a bridge may be formed, thereby degrading a reliability of a semiconductor device.
Many attempts have been made to prevent such degradation in the reliability of semiconductor devices. One proposed approach for suppressing the oxidation of tungsten has been to significantly reduce a ratio of oxygen during an oxidation process for curing the etch damage of a floating gate. But such an approach suffers from a much lower curability of the floating gate than a conventional gate re-oxidation process. Therefore, the reliability of semiconductor devices having gates cured by such a reduced-oxygen-ratio-oxidation process is still degraded.